Accurate temperature determination by thermocouple requires that the measurement junction be placed in series with a reference junction. The accuracy of the resulting temperature reading derived from the thermal EMF across the measurement junction requires knowledge of the temperature at the location of the reference junction. Prior art systems have utilized such things as ice baths or ovens to establish a stable, known environment for the reference thermocouple junction.
More recently, a temperature conditioning approach has been generated (See Hy-Cal Engineering, REF-BLOC 405 System). The reference block approach provides isothermal conditioning of the thermocouple reference junctions and a quantifiable amount of the leadwires adjacent the junction. This pre-conditions the leadwires to the same temperature as the block and prevents the leadwires from absorbing heat from the reference junctions, thereby avoiding errors. All input leads are connected via large head electrical screws to terminals on one side of the reference block and the output leads are attached to terminals on the opposite side of the block. A calibrated temperature monitoring device is attached to the reference block, resilient insulation is attached, and the entire reference block is enclosed in a weather-proof case.
With such a prior art system, the measurement thermocouple leadwires, one of which might be copper, the other constantan, would be connected to the input screws of the reference block. If both input screws are of copper, there is a reference junction formed between the copper screw head and the constantan leadwire. There are no thermocouple junctions formed at the other input screws and at the output screws since both wires and screws are copper. A common procedure is then to connect the output wires to a printed circuit edge card connector for multiplexing each channel into a meter or an analog-to-digital converter which provides computer compatible processing data. The isothermal block may or may not include an additional connection for each channel for the purpose of providing a path for an electrostatic shield wire.
Our invention improves on the above prior art system in that need for a separate reference block is eliminated. Direct connection of the incoming measurement thermocouple leads to a multiple terminal edge card connector is permitted. The reference junction is established when the dissimilar metal of the leadwire (for example, constantan) is mated to the terminal on the edge card connector. Each edge card connector accommodates several channels. By arranging several edge connected printed circuit cards in one case, a system with several dozen thermocouples can be configured. The sequencing logic can then be arranged so that temperatures of each thermocouple can be serially sampled.
With our invention the multiplicity of edge card connector terminals are made to abut an aluminum plate which presses against the insulated rear face of each connector. The aluminum plate acts as an isothermal heat sink for all of the connectors. A temperature sensitive resistor immediately adjacent the center of the aluminum plate makes it possible to measure and calibrate the temperature of all of the referenced junctions. As a result, high measurement accuracy is achieved with improved reliability.